Chromoly is an abbreviation for "chromium-molybdenum steel" (sometimes spelled "cro-mo"). Chromoly is a range of low alloy steels used, for example, to produce tubing for bicycle frames and race-car roll cages. It is not as lightweight as aluminum alloys, but has the advantages of high tensile strength and malleability. It is also easily welded and is considerably stronger and more durable than standard 1020 steel tubing.
One of the most common chromoly steels is 4130 steel, composed of 0.28 to 0.33% carbon, 0.4 to 0.6% manganese, 0.8 to 1.1% chromium, 0.15 to 0.25% molybdenum, 0.04% phosphorus, 0.04% sulfur, and 0.2 to 0.35% silicon, in addition to iron.
Although chromoly contains chromium, it does not have the corrosion resistance of stainless steel.
-Wikipedia